1. Field of the Invention
The present invention relates to integrated circuits and, more specifically, to an integrated circuit having a wiring formed of superconductive materials.
2. Description of the Prior Art
In a large scale integrated circuit such as semiconductor memory devices, the ratio of delay in the wiring portion to the delay in the entire device increases as the degree of integration becomes higher and higher. In association with the foregoing, the difference between the effect of Al and a superconductor used as wirings of the LSI is studied by simulation in an article "How much can superconductive wirings improve the speed of LSI? " in Nikkei Electronics, 1987-11-30, pp. 153-164. According to the above mentioned article, it is effective to increase the speed of operation of the device to form the wiring portion by a superconductive material in a large scale integrated circuit.
FIG. 1 shows one example of a conventionally proposed integrated circuit employing a superconductive material as a wiring. Referring to FIG. 1, an insulator 2 is arranged on a semiconductor substrate 1, and devices such as FET and the like, not shown, are formed therein. A superconductive wiring 3 is arranged on the insulator 2 with a contact portion 4 of the superconductive wiring 3 connected to said device.
In an integrated circuit having superconductive wirings such as shown in FIG. 1, Joule heat of the device itself increases the temperature during operation. Therefore, there is a possibility that the temperature of some portions of the superconductive material constituting the wiring exceed the critical temperature. The wiring would have no superconductivity at those portions and would be changed to resistance. Therefore, signal delay is generated at those portions, causing discrepancy of timing between signals in the device and therefore causing malfunctions.